Friction compensation in a minimally invasive surgical apparatus

ABSTRACT

Devices, systems, and methods for compensate for friction within powered automatic systems, particularly for telesurgery and other telepresence applications. Dynamic friction compensation may comprise applying a continuous load in the direction of movement of a joint, and static friction compensation may comprise applying alternating loads in positive and negative joint actuation directions whenever the joint velocity reading falls within a low velocity range.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/864,273,filed Jun. 8, 2004, now U.S. Pat. No. 6,974,449 which is a divisional ofU.S. patent application Ser. No. 10/402,678, filed on Mar. 27, 2003, nowU.S. Pat. No. 6,899,705, which is a divisional of U.S. patentapplication Ser. No 09/287,513, filed Apr. 7, 1999, now U.S. Pat. No.6,565,554, the full disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention is generally related to improved robotic devicesand methods, particularly for telesurgery.

Minimally invasive medical techniques are aimed at reducing the amountof extraneous tissue which is damaged during diagnostic or surgicalprocedures, thereby reducing patient recovery time, discomfort, anddeleterious side effects. Many surgeries are performed each year in theUnited States. A significant amount of these surgeries can potentiallybe performed in a minimally invasive manner. However, only a relativelysmall percentage of surgeries currently use these techniques due tolimitations in minimally invasive surgical instruments and techniquesand the additional surgical training required to master them.

Advances in minimally invasive surgical technology could dramaticallyincrease the number of surgeries performed in a minimally invasivemanner. The average length of a hospital stay for a standard surgery issignificantly longer than the average length for the equivalent surgeryperformed in a minimally invasive surgical manner. Thus, the completeadoption of minimally invasive techniques could save millions ofhospital days, and consequently millions of dollars annually in hospitalresidency costs alone. Patient recovery times, patient discomfort,surgical side effects, and time away from work are also reduced withminimally invasive surgery.

The most common form of minimally invasive surgery is endoscopy.Probably the most common form of endoscopy is laparoscopy which isminimally invasive inspection and surgery inside the abdominal cavity.In standard laparoscopic surgery, a patient's abdomen is insufflatedwith gas, and cannula sleeves are passed through small (approximately ½inch) incisions to provide entry ports for laparoscopic surgicalinstruments.

The laparoscopic surgical instruments generally include a laparoscopefor viewing the surgical field, and working tools defining endeffectors. Typical surgical end effectors include clamps, graspers,scissors, staplers, and needle holders, for example. The working toolsare similar to those used in conventional (open) surgery, except thatthe working end or end effector of each tool is separated from itshandle by, e.g., an approximately 12-inch long, extension tube.

To perform surgical procedures, the surgeon passes these working toolsor instruments through the cannula sleeves to a required internalsurgical site and manipulates them from outside the abdomen by slidingthem in and out through the cannula sleeves, rotating them in thecannula sleeves, levering (i.e., pivoting) the instruments against theabdominal wall and actuating end effectors on the distal ends of theinstruments from outside the abdomen. The instruments pivot aroundcenters defined by the incisions which extend through muscles of theabdominal wall. The surgeon monitors the procedure by means of atelevision monitor which displays an image of the surgical site via alaparoscopic camera. The laparoscopic camera is also introduced throughthe abdominal wall and into the surgical site. Similar endoscopictechniques are employed in, e.g., arthroscopy, retroperitoneoscopy,pelviscopy, nephroscopy, cystoscopy, cisternoscopy, sinoscopy,hysteroscopy, urethroscopy, and the like.

There are many disadvantages relating to current minimally invasivesurgical (MIS) technology. For example, existing MIS instruments denythe surgeon the flexibility of tool placement found in open surgery.Most current laparoscopic tools have rigid shafts and difficulty isexperienced in approaching the worksite through the small incision.Additionally, the length and construction of many endoscopic instrumentsreduces the surgeon's ability to feel forces exerted by tissues andorgans on the end effector of the associated tool. The lack of dexterityand sensitivity of endoscopic tools is a major impediment to theexpansion of minimally invasive surgery.

Minimally invasive telesurgical systems for use in surgery are beingdeveloped to increase a surgeon's dexterity as well as to allow asurgeon to operate on a patient from a remote location. Telesurgery is ageneral term for surgical systems where the surgeon uses some form ofremote control, e.g., a servomechanism or the like, to manipulatesurgical instrument movements rather than directly holding and movingthe tools by hand. In such a telesurgery system, the surgeon istypically provided with an image of the surgical site at the remotelocation. While viewing typically a three-dimensional image of thesurgical site on a suitable viewer or display, the surgeon performs thesurgical procedures on the patient by manipulating master controldevices at the remote location, which control the motion ofservomechanically operated instruments.

The servomechanism used for telesurgery will often accept input from twomaster controllers (one for each of the surgeon's hands), and mayinclude two robotic arms. Operative communication between master controland an associated arm and instrument is achieved through a controlsystem. The control system typically includes at least one processorwhich relays input commands from a master controller to an associatedarm and instrument and from the arm and instrument assembly to theassociated master controller in the case of, e.g., force feedback.

One objective of the present invention is to provide improved surgicaltechniques. Another objective is to provide improved robotic devices,systems, and methods. More specifically, it is an object of thisinvention to provide a method of compensating for friction in aminimally invasive surgical apparatus. It is a further object of theinvention to provide a control system incorporating such a method ofcompensating for friction.

BRIEF SUMMARY OF THE INVENTION

The present invention provides improved devices, systems, and methodsfor compensating for friction within powered automatic systems,particularly for telesurgery and other telepresence applications. Theinvention allows uninhibited manipulation of complex linkages, enhancingthe precision and dexterity with which jointed structures can be moved.This enhanced precision is particularly advantageous when applied to therobotic surgical systems now being developed. The friction compensationsystems of the present invention address static friction (typically byapplying a continuous load in the direction of movement of a joint) andthe often more problematic static friction (generally by applyingalternating loads in positive and negative joint actuation directions).The invention can accommodate imprecise velocity measurements byapplying an oscillating load whenever the joint velocity reading fallswithin a low velocity range. Preferably, the oscillating load isinsufficient to move the joint without additional input, andsignificantly reduces the break away input required to initiatemovement. In the exemplary embodiment, a duty cycle of the oscillatingload varies, favoring the apparent direction of movement of a velocityreading. The amplitude of the duty cycle may also vary, typicallyincreasing as the velocity reading approaches zero.

In a first aspect, the invention provides a method of compensating forfriction in an apparatus. The apparatus has at least one component thatis selectively moveable in a positive component direction, and in anegative component direction. An actuator is operatively connected tothe component. The method includes obtaining a component velocityreading, and defining a velocity reading region extending between aselected negative velocity reading and a selected positive velocityreading. A duty cycle is generated so that the duty cycle has adistribution between a positive duty cycle magnitude (corresponding to afriction compensation force in the positive component direction) and anegative duty cycle magnitude (corresponding to a friction compensationforce in the negative component direction). The distribution isdetermined by the component velocity reading when it is within thevelocity reading region. The actuator is loaded with a load defined bythe duty cycle signal.

Preferably, the duty cycle signal will have a continuous positive dutycycle magnitude (which corresponds to the friction compensation force inthe positive direction) when the component velocity reading is greaterthan the selected positive velocity reading. Similarly, the duty cyclesignal will have a continuous negative duty cycle magnitude(corresponding to the friction compensation force in the negativecomponent direction) when the component velocity reading is less thanthe selected negative velocity reading.

In the exemplary embodiment, the distribution of the duty cycle betweenthe positive and negative magnitudes is proportional to the componentvelocity reading positioned within the velocity reading region. Thepositive and negative duty cycle magnitudes may take a gravitycompensation model into account. Such a gravity compensation model maydetermine a variable gravity compensation force to applied to thecomponent, for example, to artificially balance an unbalanced linkagesystem. Such a gravity compensated system may further benefit from adetermination of a frictional compensation force corresponding to thegravity compensation force in both the positive and negative directions.In other words, in addition to compensating for friction, the method ofthe present invention may accommodate compensation factors for bothfriction and gravity, thereby simulating or approximating afriction-free balanced system, significantly enhancing the dexterity ofmovement which can be accommodated.

The selection of an appropriate oscillating frequency can significantlyenhance friction compensation provided by these methods and systems.Hence, the frequency will preferably be selected so as to besufficiently slow to enable the actuator (often including an electricalmotor and a transmission system such as gears, cables, or the like) torespond to the directing duty cycle signal by applying the desired load,and sufficiently rapid so that the load cannot actually be felt, forexample, by physically moving the joint and varying a position of aninput master control device held by a surgeon. In other words, thefrequency is preferably greater than the mechanical time constraints ofthe system, yet less than the electrical time constants of an electricalmotor used as an actuator. Preferred duty cycle frequency ranges of theexemplary telesurgical system described herein are in a range from about40 Hz to about 70 Hz, preferably being in a range from about 50 Hz toabout 60 Hz. Application of these oscillating loads can facilitatemovement of a joint in either a positive or negative direction,particularly when the velocity reading is so low that the system cannotaccurately determine whether the system is at rest, moving slowing in apositive direction, or moving slowly in a negative direction. Oncevelocity measurement readings are high enough (a given measurementreading accuracies) in a positive or negative direction, a continuous(though not necessarily constant) force in the desired direction canovercome the dynamic friction of the joint.

In yet another aspect, the invention provides a method comprisingmanipulating an input device of a robotic system with a hand of anoperator. An end effector is moved in sympathy with the manipulatingstep using a servomechanism of the robotic system. A velocity reading isobtained from a joint of the robotic system. An oscillating frictioncompensation load is applied on the joint when the velocity reading iswithin a first reading range.

Preferably, a continuous friction compensation load is applied when thereading is within a second reading range, typically above (either in thepositive or negative direction) a minimum value. The continuous load cancompensate for friction of the joint, and may vary so as to compensatefor gravity when an orientation of the joint changes. The oscillatingload similarly compensates for static friction of the joint in thepositive and negative directions, at varying points along the loadoscillation duty cycle. This method is particularly advantageous forcompensating for friction and/or gravity in a joint of the input devicefor the robotic system, particularly where the oscillating load is lessthan a static friction of the joint so that the end effector can remainstationary in the hand of the operator.

In another aspect, the invention provides a telesurgery methodcomprising directing a surgical procedure by moving an input device of atelesurgery system with a hand of an operator. Tissue is manipulated bymoving a surgical end effector in sympathy with the input device using aservomechanism of the telesurgery system. Static friction is compensatedfor in at least one joint of the robotic system by applying anoscillating load to the at least one joint when an absolute value of avelocity reading from the at least one joint is less than a velocityreading error range.

While the friction compensated joint may support the surgical endeffector, it will preferably support the input device. The oscillatingload is generally effected by applying a duty cycle to an actuator, andpreferably by altering the duty cycle in response to the velocityreading so as to facilitate movement of the joint towards the positiveorientation when the velocity reading is positive, and toward thenegative orientation when the velocity reading is negative.

In yet another aspect, the invention provides a telepresence systemcomprising a master including an input device supported by a drivenjoint. A slave includes an end effector supported by a driven joint. Acontroller couples the master to the slave. The controller directs theend effector to move in sympathy with the input device. A sensoroperatively associated with at least one of the driven joints generatesa velocity reading. An actuator drivingly engages the at least onedriven joint. The actuator applies an oscillating load on the joint tocompensate for static friction of the joint when the velocity reading iswithin a low velocity range.

Preferably, the oscillating load is insufficient to move the at leastone driven joint when the master remains stationary. In the exemplaryembodiment, the end effector comprises a surgical end effector, and theslave is adapted to manipulate the surgical end effector within aninternal surgical site through a minimally invasive surgical access.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, and withreference to the accompanying diagrammatic drawings, in which:

FIG. 1A shows a three-dimensional view of a control station of atelesurgical system in accordance with the invention;

FIG. 1B shows a three-dimensional view of a cart or trolley of thetelesurgical system, the cart carrying three robotically controlledarms, the movement of the arms being remotely controllable from thecontrol station shown in FIG. 1A;

FIG. 2A shows a side view of a robotic arm and surgical instrumentassembly;

FIG. 2B shows a three-dimensional view corresponding to FIG. 2A;

FIG. 3 shows a three-dimensional view of a surgical instrument;

FIG. 4 shows a schematic kinematic diagram corresponding to the sideview of the robotic arm shown in FIG. 2A, and indicates the arm havingbeen displaced from one position into another position;

FIG. 5 shows, at an enlarged scale, a wrist member and end effector ofthe surgical instrument shown in FIG. 3, the wrist member and endeffector being movably mounted on a working end of a shaft of thesurgical instrument;

FIG. 6A shows a three-dimensional view of a hand-held part or wristgimbal of a master control device of the telesurgical system;

FIG. 6B shows a three-dimensional view of an articulated arm portion ofthe master control device on which the hand-held part of FIG. 6A ismounted in use;

FIG. 6C shows a three-dimensional view of the master control device, thewrist gimbal of FIG. 6A shown in a mounted condition on the articulatedarm portion of FIG. 6B;

FIG. 7 shows a schematic three-dimensional drawing indicating thepositions of the end effectors relative to a viewing end of an endoscopeand the corresponding positions of master control input devices relativeto the eyes of an operator, typically a surgeon;

FIG. 8 shows a schematic graphical relationship between measuredvelocity (v) and a required force (f) to compensate for friction;

FIG. 9 shows the graphical relationship shown in FIG. 8 and one methodof compensating for friction represented in dashed lines superimposedthereon;

FIG. 10 shows the graphical relationship shown in FIG. 8 and anothermethod of compensating for friction represented in dashed linessuperimposed thereon;

FIG. 11 shows the graphical relationship shown in FIG. 8 and furtherindicates detail used to exemplify a method of compensating for frictionin accordance with the invention superimposed thereon;

FIGS. 12 to 16 show different duty cycle distributions determined byvalues derived from velocity measurements indicated in FIG. 11;

FIG. 17 shows an algorithm representing an overview of the method ofcompensating for friction in accordance with the invention;

FIG. 18 shows further detail of the algorithm shown in FIG. 17 relatingto gravity compensation;

FIG. 19 shows as an alternative to FIG. 18, further detail of thealgorithm shown in FIG. 17 relating to Coulomb friction compensation;and

FIG. 20 shows a schematic diagram exemplifying a required gravitycompensating force on a master control and how the gravity compensatingforce and consequently also frictional force, varies depending on mastercontrol position.

DETAILED DESCRIPTION OF THE INVENTION

This application is related to the following patents and patentapplications, the full disclosures of which are incorporated herein byreference: PCT International Application No. PCT/US98/19508, entitled“Robotic Apparatus”, filed on Sep. 18, 1998; U.S. Application Ser. No.60/111,713, entitled “Surgical Robotic Tools, Data Architecture, andUse”, filed on Dec. 8, 1998; U.S. Application Ser. No. 60/111,711,entitled “Image Shifting for a Telerobotic System”, filed on Dec. 8,1998; U.S. Application Ser. No. 60/111,714, entitled “Stereo ViewerSystem for Use in Telerobotic System”, filed on Dec. 8, 1998; U.S.Application Ser. No. 60/111,710, entitled “Master Having RedundantDegrees of Freedom”, filed Dec. 8, 1998; and U.S. Pat. No. 5,808,665,entitled “Endoscopic Surgical Instrument and Method for Use”, issued onSep. 15, 1998; the full disclosures of which are incorporated herein byreference.

It is to be appreciated that although the method and control system ofthe invention is described with reference to a minimally invasivesurgical apparatus in this specification, the application of theinvention is not to be limited to this apparatus only, but can be usedin any type of apparatus requiring friction compensation. Thus, theinvention may find application in the fields of satellite dish tracking,handling hazardous substances, to name but two of many possiblequalifying fields in which precisional movement is required. In somecases, it may be required to compensate for friction on a single part ofa system such as on a master controller only.

Referring to FIG. 1A of the drawings, a control station of a minimallyinvasive telesurgical system is generally indicated by reference numeral200. The control station 200 includes a viewer 202 where an image of asurgical site is displayed in use. A support 204 is provided on which anoperator, typically a surgeon, can rest his forearms while gripping twomaster controls (not shown in FIG. 1A), one in each hand. The mastercontrols are positioned in a space 206 inwardly beyond the support 204.When using the control station 200, the surgeon typically sits in achair in front of the control station 200, positions his eyes in frontof the viewer 202 and grips the master controls one in each hand whileresting his forearms on the support 204.

In FIG. 1B of the drawings, a cart or trolley of the telesurgical systemis generally indicated by reference numeral 300. In use, the cart 300 ispositioned close to a patient requiring surgery and is then normallycaused to remain stationary until a surgical procedure to be performedhas been completed. The cart 300 typically has wheels or castors torender it mobile. The control station 200 is typically positioned remotefrom the cart 300 and can be separated from the cart 300 by a greatdistance, even miles away.

Cart 300 typically carries three robotic arm assemblies. One of therobotic arm assemblies, indicated by reference numeral 302, is arrangedto hold an image capturing device 304, e.g., an endoscope, or the like.Each of the two other arm assemblies 10,10 respectively, includes asurgical instrument 14. The endoscope 304 has a viewing end 306 at aremote end of an elongate shaft thereof. It will be appreciated that theendoscope 304 has an elongate shaft to permit it to be inserted into aninternal surgical site of a patient's body. The endoscope 304 isoperatively connected to the viewer 202 to display an image captured atits viewing end 306 on the viewer 202. Each robotic arm assembly 10,10is operatively connected to one of the master controls. Thus, movementof the robotic arm assemblies 10,10 is controlled by manipulation of themaster controls. The instruments 14 of the robotic arm assemblies 10,10have end effectors which are mounted on working ends of elongate shaftsof the instruments 14. It will be appreciated that the instruments 14have elongate shafts to permit the end effectors to be inserted into aninternal surgical site of a patient's body. The end effectors areorientationally moveable relative to the ends of the shafts of theinstruments 14. The orientational movement of the end effectors are alsocontrolled by the master controls.

In FIGS. 2A and 2B of the drawings, one of the robotic arm assemblies 10is shown in greater detail.

The assembly 10 includes an articulated robotic arm 12, and the surgicalinstrument, schematically and generally indicated by reference numeral14, mounted thereon. FIG. 3 indicates the general appearance of thesurgical instrument 14 in greater detail.

In FIG. 3 the elongate shaft of the instrument 14 is indicated byreference numeral 14.1. A wrist-like mechanism, generally indicated byreference numeral 50, is located at the working end of the shaft 14.1. Ahousing 53, arranged releasably to couple the instrument 14 to therobotic arm 12, is located at an opposed end of the shaft 14.1. In FIG.2A, and when the instrument 14 is coupled or mounted on the robotic arm12, the shaft 14.1 extends along an axis indicated at 14.2. Theinstrument 14 is typically releasably mounted on a carriage 11, which isselectively driven to translate along a linear guide formation 24 of thearm 12 in the direction of arrows P.

The robotic arm 12 is typically mounted on a base by means of a bracketor mounting plate 16. The base is defined on the mobile cart or trolley300, which is normally retained in a stationary position during asurgical procedure.

The robotic arm 12 includes a cradle, generally indicated at 18, anupper arm portion 20, a forearm portion 22 and the guide formation 24.The cradle 18 is pivotally mounted on the plate 16 gimbaled fashion topermit rocking movement of the cradle in the direction of arrows 26 asshown in FIG. 2B, about a pivot axis 28. The upper arm portion 20includes link members 30, 32 and the forearm portion 22 includes linkmembers 34, 36. The link members 30, 32 are pivotally mounted on thecradle 18 and are pivotally connected to the link members 34, 36. Thelink members 34, 36 are pivotally connected to the guide formation 24.The pivotal connections between the link members 30, 32, 34, 36, thecradle 18, and the guide formation 24 are arranged to constrain therobotic arm 12 to move in a specific manner. The movement of the roboticarm 12 is illustrated schematically in FIG. 4.

With reference to FIG. 4, the solid lines schematically indicate oneposition of the robotic arm 12 and the dashed lines indicate anotherpossible position into which the arm 12 can be displaced from theposition indicated in solid lines.

It will be understood that the axis 14.2 along which the shaft 14.1 ofthe instrument 14 extends when mounted on the robotic arm 12 pivotsabout a pivot center or fulcrum 49. Thus, irrespective of the movementof the robotic arm 12, the pivot center 49 normally remains in the sameposition relative to the stationary cart 300 on which the arm 12 ismounted during a surgical procedure. In use, the pivot center 49 ispositioned at a port of entry into a patient's body when an internalsurgical procedure is to be performed. It will be appreciated that theshaft 14.1 extends through such a port of entry, the wrist-likemechanism 50 then being positioned inside the patient's body. Thus, thegeneral position of the mechanism 50 relative to the surgical site in apatient's body can be changed by movement of the arm 12. Since the pivotcenter 49 is coincident with the port of entry, such movement of the armdoes not excessively effect the surrounding tissue at the port of entry.

As can best be seen with reference to FIG. 4, the robotic arm 12provides three degrees of freedom of movement to the surgical instrument14 when mounted thereon. These degrees of freedom of movement arefirstly the gimbaled motion indicated by arrows 26, pivoting movement asindicated by arrows 27 and the linear displacement in the direction ofarrows P. Movement of the arm as indicated by arrows 26, 27 and P iscontrolled by appropriately positioned actuators, e.g., electricalmotors, which respond to inputs from an associated master controlselectively to drive the arm 12 to positions as dictated by movement ofthe master control. Appropriately positioned sensors, e.g., encoders,potentiometers, or the like, are provided on the arm to enable a controlsystem of the minimally invasive telesurgical system to determine jointpositions.

Thus, by controlling movement of the robotic arm 12, the position of theworking end of the shaft 14.1 of the instrument 14 can be varied at thesurgical site by the surgeon manipulating the associated master controlwhile viewing the responsive positional movement of the working end ofthe shaft 14.1 in the viewer 202.

Referring now to FIG. 5 of the drawings, the wrist-like mechanism 50will now be described in greater detail. In FIG. 5, the working end ofthe shaft 14.1 is indicated at 14.3. The wrist-like mechanism 50includes a wrist member 52. One end portion of the wrist member 52 ispivotally mounted in a clevis, generally indicated at 17, on the end14.3 of the shaft 14.1 by means of a pivotal connection 54. The wristmember 52 can pivot in the direction of arrows 56 about the pivotalconnection 54. An end effector, generally indicated by reference numeral58, is pivotally mounted on an opposed end of the wrist member 52. Theend effector 58 is in the form of, e.g., a clip applier for anchoringclips during a surgical procedure. Accordingly, the end effector 58 hastwo parts 58.1, 58.2 together defining a jaw-like arrangement. It willbe appreciated that the end effector can be in the form of any requiredsurgical tool having two members or fingers which pivot relative to eachother, such as scissors, pliers for use as needle drivers, or the like.Instead, it can include a single working member, e.g., a scalpel,cautery electrode, or the like. When a tool other than a clip applier isrequired during the surgical procedure, the tool 14 is simply removedfrom its associated arm and replaced with an instrument bearing therequired end effector, e.g., a scissors, or pliers, or the like.

The end effector 58 is pivotally mounted in a clevis, generallyindicated by reference numeral 19, on an opposed end of the wrist member52, by means of a pivotal connection 60. It will be appreciated thatfree ends 11, 13 of the parts 58.1, 58.2 are angularly displaceableabout the pivotal connection 60 toward and away from each other asindicated by arrows 62, 63. It will further be appreciated that themembers 58.1, 58.2 can be displaced angularly about the pivotalconnection 60 to change the orientation of the end effector 58 as awhole, relative to the wrist member 52. Thus, each part 58.1, 58.2 isangularly displaceable about the pivotal connection 60 independently ofthe other, so that the end effector 58, as a whole, is angularlydisplaceable about the pivotal connection 60 as indicated in dashedlines in FIG. 5. Furthermore, the shaft 14.1 is rotatably mounted on thehousing 53 for rotation as indicated by the arrows 59. Thus, the endeffector 58 has three orientational degrees of freedom of movementrelative to the working end 14.3, namely, rotation about the axis 14.2as indicated by arrows 59, angular displacement as a whole about thepivot 60 and angular displacement about the pivot 54 as indicated byarrows 56. It will be appreciated that orientational movement of the endeffector 58 is controlled by appropriately positioned electrical motorswhich respond to inputs from the associated master control to drive theend effector 58 to a desired orientation as dictated by movement of themaster control. Furthermore, appropriately positioned sensors, e.g.,encoders, or potentiometers, or the like, are provided to permit thecontrol system of the minimally invasive telesurgical system todetermine joint positions.

In use, and as schematically indicated in FIG. 7 of the drawings, thesurgeon views the surgical site through the viewer 202. The end effector58 carried on each arm 12 is caused to perform movements and actions inresponse to movement and action inputs of its associated master control.It will be appreciated that during a surgical procedure responsivemovement of the robotic arm 12 on which the surgical instrument 14 ismounted causes the end effector to vary its position at the surgicalsite whilst responsive movement of the end effector relative to the end14.3 of the shaft 14.1 causes its orientation to vary relative to theend 14.3 of the shaft 14.1. Naturally, during the course of the surgicalprocedure the orientation and position of the end effector is constantlychanging in response to master control inputs. The images of the endeffectors 58 are captured by the endoscope together with the surgicalsite and are displayed on the viewer 202 so that the surgeon sees thepositional and orientational movements and actions of the end effectors58 as he or she controls such movements and actions by means of themaster control devices.

An example of one of the master control devices is shown in FIG. 6C andis generally indicated by reference numeral 700. The master control 700includes a hand-held part or wrist gimbal 699 and an articulated armportion 712. The hand-held part 699 will now be described in greaterdetail with reference to FIG. 6A.

The part 699 has an articulated arm portion including a plurality ofmembers or links 702 connected together by joints 704. The surgeon gripsthe part 699 by positioning his or her thumb and index finger over apincher formation 706 of the part 699. The surgeon's thumb and indexfinger are typically held on the pincher formation 706 by straps (notshown) threaded through slots 710. The joints of the part 699 areoperatively connected to electric motors to provide for, e.g., forcefeedback, gravity compensation, and/or the like. Furthermore,appropriately positioned sensors, e.g., encoders, or potentiometers, orthe like, are positioned on each joint of the part 699, so as to enablejoint positions of the part 699 to be determined by the control system.

The part 699 is mounted on the articulated arm portion 712 indicated inFIG. 6B. Reference numeral 4 in FIGS. 6A and 6B indicates the positionsat which the part 699 and the articulated arm 712 are connectedtogether. When connected together, the part 699 can displace angularlyabout an axis at 4.

Referring now to FIG. 6B, the articulated arm 712 includes a pluralityof links 714 connected together at joints 716. Articulated arm 712 mayhave appropriately positioned electric motors to provide for, e.g.,force feedback, gravity compensation, and/or the like. Furthermore,appropriately positioned sensors, e.g., encoders, or potentiometers, orthe like, are positioned on the joints 716 so as to enable jointpositions of the master control to be determined by the control system.

When the pincher formation 706 is squeezed between the thumb and indexfinger, the fingers of the end effector 58 close. When the thumb andindex finger are moved apart the fingers 58.1, 58.2 of the end effector58 move apart in sympathy with the moving apart of the pincher formation706. To cause the orientation of the end effector 58 to change, thesurgeon simply causes the pincher formation 706 to change itsorientation relative to the end of the articulated arm portion 712. Tocause the position of the end effector 58 to change, the surgeon simplymoves the pincher formation 706 to cause the position of the articulatedarm portion 712 to change.

The electric motors and sensors associated with each robotic arm 12 andthe surgical instrument 14 mounted thereon, and the electric motors andthe sensors associated with each master control device 700, namely thepart 699 and the articulated arm portion 712, are operatively linked inthe control system (not shown). The control system typically includes atleast one processor for effecting control between master control deviceinput and responsive robotic arm and surgical instrument output and foreffecting control between robotic arm and surgical instrument input andresponsive master control output in the case of, e.g., force feedback.

As can best be seen in FIG. 6C, each master control device 700 istypically mounted on the control station 200 by means of a pivotalconnection, as indicated at 717. As mentioned hereinbefore, tomanipulate each master control device 700, the surgeon positions histhumb and index finger over the pincher formation 706. The pincherformation 706 is positioned at a free end of the articulated arm portionof the part 699, which in turn is positioned on a free end of thearticulated arm 712. It will be appreciated that the master controldevice 700 has a center of gravity normally removed from the verticalrelative to its pivotal connection 717 on the control station 200. Thus,should the surgeon let go of the pincher formation 706, the mastercontrol device 700 would drop due to gravity. It has been found thatproviding the master controls 700, 700 with gravity compensation so thatwhenever the surgeon lets go of the pincher formations 706, 706, themaster controls 700, 700 remain at their positions and orientations isbeneficial. Furthermore, since performing surgical procedures involvesprecision movements, it is beneficial that the surgeon does not need tocope with a weighted feeling when gripping the pincher formations706,706 of the master controls 700, 700. Thus, the control system of thetelesurgical minimally invasive system is arranged to provide gravitycompensation to the master control devices 700, 700. This gravitycompensation can be achieved passively by use of counterbalancers,and/or springs, and/or the like, and/or actively by appropriateapplication of forces or torques on the motors operatively associatedwith each master control 700. In the present case, the gravitycompensation is achieved actively by means of appropriate compensatingtorques on motors associated with each master control 700.

It will be appreciated that operative connection between the electricalmotors and the master controls 700, 700, is typically achieved by meansof transmissional components. These transmissional components typicallyinclude gear trains. Naturally, other transmissional components such aspulley and cable arrangements, and/or the like, can be used instead, orin addition. Regardless of the specific transmission used, thesecomponents will generally induce both static and dynamic friction in thetelesurgical system.

It has been found that in providing gravity compensation, the geartrains between the motors and the master controls are typically underload. This increases the frictional forces between meshing gears andleads to increased friction when the master control is moved or urged tomove by the surgeon. It has been found that the increase in frictionalforces, due to gravity compensation in particular, renders mastercontrol movement uncomfortable and unpleasant (and may lead to imprecisemovements) due to hysteresis.

Referring to FIG. 8 of the drawings, a typical graphical relationshipbetween velocity and a desired force for compensation of friction isindicated by reference numeral 510. Velocity is indicated on thehorizontally extending axis and the required compensating frictionalforce is indicated on the vertically extending axis. To the left of thevertical axis a force in an arbitrary negative direction is indicated,and to the right of the vertical axis a force in an arbitrary positivedirection is indicated. When movement is to be induced from a restposition, the force required to induce movement from rest is normallyhigher than that required to maintain movement after movement isinitiated. This characteristic of friction is indicated by the opposed“spikes” at 512 in FIG. 8, and is referred to as “stiction.” The spikeshave been indicated in extended fashion along the velocity axis for thesake of clarity. However, it is to be appreciated that the spikesnormally occur on the force axis and need not extend along the velocityaxis as indicated. Note that the dynamic friction forces may not beperfectly constant, but may vary with velocity. When movement isinitiated friction can readily be compensated for by applying acorresponding compensating force. However, to achieve adequate frictioncompensation when initiating movement from rest, or when changingdirection, is more problematic.

A first friction compensation technique can best be described withreference to the following simple electromechanical system, by way ofexample. The example of the electromechanical system includes a motorand an articulated arm. The motor is arranged to drive the articulatedarm through a transmission arrangement, e.g., a gear train, or the like.For the sake of this example, the graphical relationship betweenvelocity and frictional force shown in FIG. 8 represents the mechanicalfriction in the electromechanical system as a function of velocity. Itis generally desirable to compensate for this friction within theelectromechanical system, as the friction can be distracting to theoperator, limiting the operator's dexterity and effectiveness.

One method of compensating for friction, in particular for compensatingfor stiction, when the arm of the example is at rest, is to inhibit theelectromechanical system from ever fully being at rest. This methodincludes cyclically supplying a current to the motor to prevent theelectromechanical system from fully coming to rest. Thus, the motor iscaused cyclically to move angularly in opposed directions. Thus acyclical torque is supplied to the motor causing the slave to oscillate.This method is referred to as “dithering.”

Although this method inhibits the system from coming to rest and thusobviates stiction when movement is to be induced from a rest position,it has been found that dithering causes vibration in the system which isuncomfortable in some applications, particularly in minimally invasivesurgical procedures. Furthermore, dithering can lead to excessive wearand ultimately damage to the apparatus.

Another method of compensating for friction is represented in FIG. 9.This method involves supplying a force of a magnitude approximating thefrictional force in the system whenever it is in motion. This type ofcompensation is referred to as “Coulomb” friction compensation. Such aforce is induced in the electromechanical system by means of motortorque of a magnitude corresponding to the frictional force required tomaintain movement in a specific direction after movement is achieved inthat direction. The compensating force is indicated in dashed lines byreference numeral 520 with the sign of the compensating force beingdetermined by the sign of the measured velocity.

This method also does not make allowance for the spikes at 512. Thus, adegree of “sticking,” or stiction, is still felt when movement isinitiated. Since it is difficult to measure velocity accurately when asystem is at rest due to measurement inaccuracies, noise, and the like,it is problematic in applying the compensating force in the correctdirection. Accordingly, when movement is to be initiated in onedirection from rest, the system could be measuring a velocity in theopposed direction, in which case the compensating force is applied inthe same direction as the frictional force thus aggravating stiction.Should the velocity reading fluctuate at zero, a compensating forcewhich fluctuates in opposed directions is generated which introducesunpredictable energy into the system tending to destabilize it andgiving it an active “feel.”

Another method of compensating for friction is indicated in FIG. 10 indashed lines generally indicated by reference numeral 530. This methodis similar to the “Coulomb” type of compensation. However, inaccuracy inmeasurement around a zero velocity reading is compensated for byslanting the compensation across zero velocity. Although this methodcompensates for system uncertainty at zero velocity, it does not alwaysaccurately compensate for friction forces at low velocity, norcompensate for stiction when movement is to be initiated from rest.Thus, stiction is normally still present.

The preferred method of compensating for friction in accordance with theinvention will now be described with particular reference tocompensating for friction in a gear train of one of the master controls700, 700 due to gravity compensation. It will be appreciated that thedescription which follows is by way of example only and that the methodof compensating for friction is not limited to this application only,but can be readily adapted to compensate for other sources of frictionsuch as, e.g., at pivotal joints, between components which translaterelative to each other, and/or the like. Furthermore, the method canenjoy universal application to compensate for friction in any systemwhether to compensate for friction due to gravity compensation or merelyto compensate for friction in general irrespective of the source. Forexample, gear train loads imposed for purposes other than gravitycompensation, for example, by a controller other than gravitycontroller, may induce friction that can be compensated for.

The method of compensating for friction in accordance with the inventioncan be understood with reference to a single joint of the master control700, for example the joint 704B in FIG. 6B of the drawings, and anelectrical motor associated with that joint through a gear train. Itwill be appreciated that friction compensation can be provided for eachjoint of the master control 700.

Referring to FIG. 11 of the drawings, a graphical relationship betweenangular velocity (v) of the joint 704B (as measured by the controlsystem) and the force (f) which will compensate for force in the geartrain associated with the joint 704B is generally indicated by referencenumeral 110. Velocity (v) extends along the horizontal axis and therequired force (f) to compensate for friction in the gear train extendsalong the vertical axis.

It has been found that when the arm members or links 702A connectedtogether by means of the joint 704B are in a stationary positionrelative to one another, and the surgeon wishes to move the mastercontrol 700 in a manner initiating movement of the arm members 702Aabout the joint 704B, friction is particularly evident. The reason forthis is that the force required to overcome friction from a stationaryposition is higher than the force required to maintain movement aftermovement is achieved. As soon as movement is achieved, the frictionalforce decreases and then stays approximately constant as velocityincreases. This phenomenon is schematically indicated by the opposedspikes at 112 around the zero velocity region and is termed stiction.Once movement is achieved, the frictional force requiring compensationis generally constant as indicated by the straight line portions 114.

It will be appreciated that movement of the pincher formation 706 isachieved through a plurality of joints, namely joints 704, 716 and 717.Thus, during any given pincher formation movement any one or more of thejoints 704, 716, 717 may be at rest so that initiating movement about anarbitrary stationary joint or joints may be required while the pincherformation is actually moving. Thus, since there are a plurality ofjoints, stiction has a cumulative effect which renders precise movementof the master 700 difficult to maintain even while the pincher formation706 is actually moving. When the pincher formation 706 is to be movedvery slowly, stiction of any one or more of the joints 704, 716 and 717is particularly problematic and renders precise movement of the pincherformation 706 (and also responsive movement by the end effector 58)difficult to maintain. In fact, smooth motion of pincher formation 706will involve directional changes of some of the joints. This can lead tosignificant changes in the cumulative friction force, again renderingprecise movements difficult to maintain. To overcome or compensate forstiction and the differences between static and dynamic frictionalforces is particularly advantageous, since slow precise movements areoften employed during a surgical procedure. Compensating for stictionand the static/dynamic differential is also particularly problematic.One reason for this is that available sensors used to measure angularvelocity are not entirely accurate so that precisely measuring zerovelocity of the joint when at rest is difficult. Another reason is thatnoise may be superimposed on the sensor signal which further aggravatesthe problem of measuring zero velocity when the joint is at rest. Thus,when the joint is at rest, the sensors can be registering movement and,consequently, apparent velocity.

The joint can move in an arbitrary positive and an arbitrary negativedirection. The velocity reading may have a negative value, a positivevalue, or may be fluctuating about the zero velocity reading when thejoint is at rest due to the noise and measurement inaccuracies. If thevelocity reading is used to determine a frictional compensation force,it is difficult to determine when and in what direction to apply thefrictional force since the velocity reading does not correspond with theactual velocity of the joint particularly when the joint is at rest.Even with an accurate velocity measurement, using a sensor whichaccurately measures zero velocity when the joint is actually at rest, itwould still be problematic to apply a frictional compensation force tocompensate for stiction since it is not easy to anticipate in which ofthe arbitrary positive and negative directions the joint will be moved.

To overcome these problems, and to compensate for stiction in particularwhilst accommodating measurement inaccuracies, a velocity regionindicated between the arrows X-X is chosen, such that if the velocityreading is within this region, a cyclical torque, varying in a positiveand a negative direction is supplied to the motor so that irrespectiveof the direction in which movement is to be initiated from rest, afriction compensation torque is provided at least part of the time. Thiswill be described in greater detail below.

The indicated velocity region X-X can be chosen based on measurementaccuracy such that outside the region the joint is actually movingwhilst inside the region the joint could either be moving very slowly ineither direction or may be stationary. Outside the region X-X, it isassumed that the velocity reading does indicate joint movement in acorrect direction and that movement has been initiated. A uniformcompensating torque is then applied corresponding with the constantfriction experienced when movement is achieved, as will be described ingreater detail herein below.

Still referring to FIG. 11 of the drawings, the control system of theinvention is arranged to generate compensating values determined by thevelocity reading within the region X-X. This can best be explained bymeans of the slanted dashed line in FIG. 11. The slanted dashed line DLextends between opposed intersections of the chosen velocity readingregion X-X, and the required force for compensating for friction.Naturally, the slanted line need not be linear but could be rounded atits corners, and/or the like. Furthermore, the width of the regionbetween X-X can be tailored to suit the system friction characteristics.

The friction compensating force values along dashed line DL can berepresented as percentages for generating a duty cycle appropriate to ameasured velocity. Should the velocity reading be at +v1 a force valueof 100% is generated. Similarly, if the velocity reading is at −v1, avalue of 0% is generated. In similar fashion a specific value rangingbetween 0% and 100% is generated depending upon the measured velocityreading position between +v1 and −v1.

The value thus generated can be used to determine a duty cycle signaldistribution between the arbitrary positive and the arbitrary negativedirection of movement about the joint 704B. Thus, where a value of 0% isgenerated, the reading then being negative, in other words, in anarbitrary negative direction, a duty cycle as indicated in FIG. 12 isgenerated. The distribution of the duty cycle in FIG. 12 iscorrespondingly fully negative, or 100% negative. The region X-X can bechosen such that at this point, taking noise and measurementinaccuracies into account, the master may be either about to actuallymove in the negative direction or may already be moving in the negativedirection.

Similarly, should a value of 20% be generated, for example, a duty cycleas indicated in FIG. 13 is generated. The distribution of the duty cyclein FIG. 13 is correspondingly 20% positive and 80% negative.

Should a value of 50% be generated, a duty cycle as indicated in FIG. 14is generated. The distribution of the duty cycle in FIG. 14 iscorrespondingly 50% positive and 50% negative.

Similarly, should a value of 80% be generated, a duty cycle as indicatedin FIG. 15 is generated. The distribution of the duty cycle in FIG. 15is correspondingly 80% positive and 20% negative.

In the case where a value of 100% is generated, a duty cycle asindicated in FIG. 16 is generated. The distribution of the duty cycle inFIG. 16 is correspondingly fully positive, or 100% positive. At thispoint, taking noise and measurement inaccuracies into account, themaster can be either about to actually move in the positive direction ormay already be moving in the positive direction.

It will be appreciated that the duty cycles shown need not necessarilyhave generally rectangular waveforms.

It will further be appreciated that when the joint is at rest, thevelocity reading is typically fluctuating within the X-X region so thatthe duty cycle distribution is continually varying.

The method of compensating for friction will now be described in furtherdetail with reference to FIG. 17.

In FIG. 17, a block diagram indicating steps corresponding to the methodof compensating for friction in accordance with the invention isgenerally indicated by reference numeral 410.

The velocity readings as described above are indicated at 412. Thecompensating values determined from the velocity readings is indicatedat 414. The compensating values are input to a duty cycle generator suchas a PWM generator at 416. The resultant duty cycle signal distributionis output from the PWM generator.

It will be appreciated that the steps from 412 to 416 are used todetermine only the percentage distribution of the duty cycle signalbetween the arbitrary negative and positive joint movement directions.This determination is directly related to the velocity measurementsbetween arrows XX. The determination of the amplitude or magnitude ofthe duty cycle signal will now be described.

As mentioned earlier, the control system compensates for gravity. Themaster control 700 is moveable about a pivot at 717 and the pincherformation 706 is connected to the pivot 717 through the joints 704, 716and the intervening arm members. The master control 700 as a whole isthus displaceable about the pivot 717. A horizontal component of thecenter of gravity varies as the pincher formation 706 is displaced.Accordingly, the torque supplied to an electrical motor operativelyassociated with the master control 700 and which balances andcompensates for gravity also varies. Thus, the gravity compensatingtorque on the electrical motor is determined in part by the position ofthe center of gravity. This is indicated schematically in FIG. 20 of thedrawings by way of example. In FIG. 20, it can be seen that the torquerequired on a motor M1 to hold an arm A10 in a position as indicated insolid lines to compensate for gravity is greater than that required tohold the arm in the position indicated in dashed lines. A similarprincipal applies for each joint of the master control 700. Naturally,the higher the gravity compensating torque supplied to the motor, thehigher the transmission loading on the associated gear train andtherefor the higher the frictional force and vice versa.

Each joint 704, 716, 717 may have an actuator, e.g., electric motor,operatively associated therewith to provide for, e.g., force feedback.Furthermore, for each joint employing gravity compensation, acorresponding gravity compensating torque is supplied to the motoroperatively associated therewith. The gravity compensation torquemagnitude varies depending on master control position. The motoroperatively associated with each joint employing gravity compensationcan be provided with a friction compensation torque in accordance withthe method of the invention. The friction compensation torque magnitudeapplied to a particular joint varies in accordance with the gravitycompensation torque. It will be appreciated that the effects of frictioncan be negligible on some of the joints. Hence, friction compensationmay not be provided for all joints of the master and/or slave.

The friction compensation loads induced by the gravity compensationsystem need not, and generally will not, be applied separately. Theexemplary friction compensation system described herein incorporates thegravity model, so that the gravity compensation torques become part ofthe load applied by the friction compensation system. Alternatively,separate gravity compensation and friction compensation loads might bemaintained.

Referring once again to FIG. 17 of the drawings, a gravity compensatingmodel is indicated at 418 whereby gravity compensation forces for thejoints requiring gravity compensation are determined. For each of thejoints 704, 716, 717 employing gravity compensation, the gravitycompensation model determines the torque which can hold the part of themaster control 700 extending from that joint in the direction of thepincher formation 706 in a stationary position. Naturally, this torquevaries for each joint in sympathy with positional variation of thatjoint as the master control 700 is moved from one position to a nextposition.

Referring now to FIG. 18 of the drawings, the gravity and friction(efficiency) model 418 will now be described in greater detail. From thegravity model, indicated at 419, the magnitude of a desired gravitycompensating force for the joint, e.g., joint 704B, is determined. Thegravity compensating force is then forwarded to a friction compensationdetermining block 451 for determining friction compensation in thearbitrary positive joint movement direction as indicated by line 452.The gravity compensation force is also forwarded to a frictioncompensation determining block 453 for determining friction compensationin the arbitrary negative joint movement direction as indicated by line454.

In the block 451, the magnitude of the gravity compensating force isrepresented along a horizontally extending axis and the correspondingrequired frictional compensating force for the positive joint movementdirection is represented along a vertically extending axis. Thecorresponding frictional compensating force is determined taking thegear train efficiency into account as indicated by the lines 1/eff andeff, respectively (eff being efficiency, typically less than 1).

In similar fashion, in the block 453, the magnitude of the gravitycompensating force is represented along a horizontally extending axisand the corresponding required frictional compensating force for thenegative joint movement direction is represented along a verticallyextending axis. The corresponding frictional compensating force isdetermined taking the gear train efficiency into account as indicated bythe lines 1/eff and eff, respectively.

The magnitudes of the frictional compensating forces in respectively thepositive and the negative joint movement directions determined in theblocks 451, 453 represent the magnitudes of the frictional forces inrespectively the positive and negative joint movement directions aftermovement of the joint has been initiated. Thus, they correspond with thelines 114 in FIG. 11 of the drawings.

The magnitude of these forces are used to determine the amplitude of theduty cycle signal at 416. Thus, from 414 the percentage distributionbetween the arbitrary positive and negative directions were determined,and from the gravity model at 418, the magnitude or amplitude of theduty cycle signal is determined for each arbitrary positive and negativejoint movement direction. It will be appreciated that these magnitudescorrespond to dynamic friction compensating forces. Depending on actualjoint position, these compensating forces can be dissimilar.

As mentioned earlier, overcoming friction when at rest involves a higherforce than is applied to maintain movement. This characteristic offriction is compensated for at 420 when the velocity reading lies in theregion Y-Y as indicated (also designated as the region between −V2 andV2). The force which can cause an object, in this case the meshing gearsof the gear train, to break away from a rest position is typically somefactor higher than 1, often being about 1.6 times the force to maintainmovement after movement is achieved. This factor can vary depending onthe application. In this case, the factor or ratio corresponds to therelationship between the force which will overcome friction in the geartrain when at rest and to maintain movement in the gear train oncemovement has been initiated. More specifically, the ratio corresponds tothe change in efficiency of the gear train when at rest versus when inmotion. It will be appreciated that at 420, the ratio and effectiverange Y-Y can be tailored to suit a specific application. The range Y-Ycould correspond with the range X-X, for example.

Referring now to 420 in greater detail, and assuming the region Y-Ycorresponds with the region X-X, at 0% and 100% values, a factor of 1 isgenerated. At a 50% value a maximum factor is generated. Between 50% and100% and between 50% and 0% a linear relationship between the maximumfactor value, in one example 1.6, and the minimum factor value, namely1, is established. Thus, at a value of 75% or 25% a factor of 1.3 wouldbe generated. It will be appreciated that the relationship need notnecessarily be linear.

The factor ranging between 1 and the maximum factor determined at 420from the velocity reading is then output or forwarded to factoring oradjusting blocks at 422 and 421, respectively.

The friction compensation force for movement in the positive jointdirection is input to the block 422 as indicated by line 424. In theblock 422, this friction compensation value is indicated along thehorizontal axis. The actual friction compensation force magnitude tocompensate for stictions is indicated along the vertical axis. The valueof the factor is indicated by the letters “fac.” This value determinesthe relationship between the actual required friction compensationforces and the friction force requiring compensation when movement inthe positive joint direction is achieved. Thus, the value fac determinesthe gradient of the lines indicated by fac and 1/fac, respectively.Naturally, when fac=1, the lines fac and 1/fac extend at 45° resultingin the actual required friction being equal to the friction requiringcompensation. This corresponds with a condition in which the velocityreading is outside or equal to the outer limits of the Y-Y region. Itwill be appreciated that at 421, a similar adjustment takes place forfriction compensation force in the negative direction.

It will be appreciated that a larger force to compensate for friction inone direction may be required than in the opposed direction, inparticular because our compensation torque here indicates both frictioncompensation torque and gravity compensation torque. This depends on theactual position of the joint. Normally, to cause the arm memberextending from the joint toward the pincher formation 706 to move in anoperatively downward direction requires less friction compensationtorque than moving it in an operatively upward direction. Thus, shouldthe arbitrary positive joint movement direction correspond with anupward movement, a greater frictional compensating force is requiredthan that in the arbitrary negative direction, and vice versa. Thus, theamplitude of the duty cycle can be higher or lower on the positive sidethan the negative side depending on the position of the joint, andwhether the arbitrary positive joint movement direction corresponds withan upward or downward movement of the arm member extending from thejoint. Indeed, the “positive” compensation load need not be in thepositive direction and the “negative” load need not be in the negativedirection, although the positive load will be greater than or equal tothe negative load.

After the friction compensation force magnitudes have been determined inthis manner, they are forwarded to the PWM signal generator at 416 asindicated by lines 462 and 464, respectively. At the PWM signalgenerator, the force magnitudes are combined with the duty cycledistribution signal determined at 414 to determine a resultant dutycycle signal as indicated at 466. The resultant duty cycle signal 466 isthen passed from the PWM signal generator along line 468.

The duty cycle signal thus determined by the PWM signal generator 416 bycombining outputs from 414, 421 and 422 is then passed to an amplifierso that the required electrical current can be passed to the electricalmotor operatively associated with the joint 704B so as to generatecorresponding cyclical torques on that motor.

The frequency of the duty cycle output from 416 is predetermined so asto be low enough to enable the electrical motor to respond and highenough so as not to be felt mechanically. Thus, the frequency is greaterthan the mechanical time constants of the system yet less than theelectrical time constants of the electric motor. A suitable frequency inthe exemplary telesurgical system falls in the range between 40 Hz to 70Hz, preferably about 55 Hz.

It will be appreciated that where it is possible accurately to read zerovelocity when the master control 700 is at rest, the above method ofcompensating for friction can also be used. For example, when the mastercontrol 700 is stationary and a zero velocity reading is measured, aduty cycle is forwarded to the motors, the duty cycle having a magnitudecorresponding to the required frictional compensating force and having a50% distribution. Thus, when an external force is applied to the handcontrol by the surgeon in a specific direction, a friction compensatingforce is delivered 50% of the time to assist in initiating movement ofthe master control 700, thus to compensate for stiction. As movement isthen induced and the velocity reading increases in a specific direction,the distribution of the cycle changes in a direction corresponding tothe direction of movement of the master control. Eventually, when themaster control is being moved at a velocity corresponding to a velocityreading outside the range XX, the compensating force, or torque to themotors, is distributed 100% in a direction corresponding to thedirection of movement of the master control. The duty cycle has apredetermined frequency so that, irrespective of the direction ofrequired movement induced on the master control 700 when the mastercontrol 700 is moved, e.g., by the surgeon's hand, a correspondingfriction compensating force is supplied at a percentage of the timedetermined by the velocity reading. The effect of this is that duringmovement initiation, the sticking sensation is compensated for. Thisenables smooth precision movements to be induced on the master controlwithout sticking, particularly at small velocities.

As mentioned, the method of compensating for friction is not limited tofriction resulting from gravity compensation. In other words, gravitymodel might be replaced by some other controller determining torques tobe applied to the motors for another purpose. The method can be used tocompensate for friction per se.

Referring now to FIG. 19, a method of compensating for friction asapplied to friction per se will now be described. The method is similarto the method described above with reference to gravity compensation.However, in this case, the gravity model is replaced by a Coulombfriction model which provides a fixed compensating friction value in thearbitrary positive and negative joint movement directions. The fixedcompensating friction can be set to correspond with an actual constantfriction value for friction compensation as defined by actual systemparameters. The adjustment factor simply may multiply these fixed valuesin 421 and 422. This method can be used to overcome actual friction inthe joint itself, for example, should the friction in the joint requirecompensation. In other respects, the method of compensating forfriction, and stiction, as discussed above applies. Hence, this methodcan be combined with the system described above or with another gravityand/or friction model using appropriate adjustments 421 and 422.

While the exemplary embodiment has been described in some detail, by wayof example and for clarity of understanding, a variety of changes andmodifications will be obvious to those of skill in the art. Hence, thescope of the present invention is limited solely by the appended claims.

1. A method for friction compensation of a movable mechanical part in amedical robotic system, comprising: determining a velocity reading forthe movable mechanical part; and applying an alternating direction,friction compensation load on the movable mechanical part when thevelocity reading is within a first reading range.
 2. A method as claimedin claim 1, further comprising applying a continuous frictioncompensation load on the movable mechanical part when the velocity iswithin a second reading range.
 3. A method as claimed in claim 2,wherein the continuous load applying step is applied in a firstdirection so as to compensate for dynamic friction of the movablemechanical part when the part moves in the first direction, and isapplied in a second direction so as to compensate for dynamic frictionof the movable mechanical part when the part moves in the seconddirection.
 4. A method as claimed in claim 3, further comprisingcompensating for gravity by varying the continuous load when anorientation of the movable mechanical part changes.
 5. A method asclaimed in claim 1, wherein the application of the alternatingdirection, friction compensation load is performed so that a first loadapplied in a first direction compensates for static friction of themovable mechanical part in the first direction and a second load appliedin a second direction compensates for static friction of the movablemechanical part in the second direction.
 6. A method as claimed in claim5, wherein the alternating direction, friction compensation load is anoscillating load, and further comprising varying a duty cycle of theoscillating load in response to the velocity reading within the firstrange.
 7. A method as claimed in claim 2, wherein the load applyingsteps are performed by energizing an actuator drivingly engaging themovable mechanical part.
 8. A method as claimed in claim 1, wherein thealternating direction, friction compensation load is less than a staticfriction of the movable mechanical part so that the part remainssubstantially stationary until an externally applied force sufficient tomove the movable mechanical part is applied.
 9. A method as claimed inclaim 1 wherein the movable mechanical part is a joint.
 10. A method asclaimed in claim 9 wherein the joint is included in an input devicemanipulatable by a surgeon to move an end effector of a surgical tool ofthe medical robotic system during a surgical procedure.